The parent invention of the above noted ""747 application relates to a high temperature, composite ceramic filter having particular use as a diesel engine exhaust filter, and to a method of forming the same.
As regulatory agencies have recently mandated the reduction of particulate emissions in diesel engines, there has been increased activity in the development of exhaust gas filters for diesel engines. A typical exhaust filter will trap the particulate material contained in the exhaust stream, and to prevent clogging of the filter and the resultant increase of load on the engine due to increased back pressure, the particulate material is then burned from the filter.
As a diesel engine exhaust filter must have high temperature resistance and durability, it has been proposed to utilize ceramic materials as the filter media. For example, a ceramic filter for use in gas turbine engines is described in U.S. Pat. No. 4,017,347. In this patent, a ceramic tape or sheet is prepared from a slurry containing ceramic particles and a two-resin binder system composed of a thermoplastic resin and a thermosetting resin. The tape is formed into a filter structure and the structure is then fired to sinter the ceramic particles and burn out the organic constituents, thus producing a sintered ceramic cellular structure.
U.S. Pat. No. 5,322,537 discloses an exhaust gas filter for diesel engines composed of ceramic fibers, such as alumina-silicate fibers, and an inorganic binder having a softening temperature lower than that of the ceramic fiber.
U.S. Pat. No. 4,652,286 also describes a ceramic exhaust filter for diesel engines having a corrugated or honeycomb structure made of sheets consisting of ceramic fibers and a fire clay binder.
A variety of structures are used to control diesel exhaust emissions, including extruded monolithic structures and corrugated spiral structures. These suffer a variety of shortcomings, depending on the specific design, including high thermal mass, high restriction, low capacity, and poor durability. Diesel emission control filters (DECF) including diesel particulate filters (DPF), diesel oxidation catalyst (DOC), and other types of flow-through and wall-flow filters, typically use a cylindrical geometry with flow in an axial direction through channels, also oriented axially. To create the axial channels, the diesel emission control filter element may be an extruded honeycomb monolith, or a spiral wound structure of alternating layers of corrugated and flat filter media, or a spiral wound structure of alternating layers of pleated and flat filter media. The present invention provides improvements in the latter type.
The noted parent invention is directed to a high temperature composite ceramic filter having particular use as a diesel engine exhaust filter, and to a method of making the same.
In carrying out the parent invention, an aqueous slurry is initially produced containing random length ceramic fibers, organic fibers and a water soluble thermoplastic binder. The ceramic fibers, such as alumina, have high temperature resistance, being stable to temperatures above 1000xc2x0 C. The organic fibers can take the form of natural or synthetic materials.
The slurry is formed into a paper-like sheet by conventional papermaking techniques, and the sheet is subsequently dried to evaporate the water and provide a dry flexible sheet.
The sheet is then formed into a green three-dimensional article suitable for filtering. Preferably, the final shape is that of a spirally wound, honeycomb element, composed of flat and corrugated layers, with a colloidal solution of a ceramic material used as an adhesive to join the sheets together along contiguous areas. Opposite ends of alternate channels within the honeycomb structure are sealed by a high temperature cement.
As a feature of the parent invention, the green filter structure is coated with an intermediate binder to increase its temperature stability. In one form of the parent invention, the intermediate binder consists of a solvent solution of an uncured thermosetting resin, such as a phenol-formaldehyde resin. The coated part is then air dried and heated to a temperature generally in the range of about 150xc2x0 C. to 250xc2x0 C. to crosslink the resin and rigidify the structure. The structure is then pyrolyzed at a temperature generally in the range of 900xc2x0 C. to 1000xc2x0 C. in an inert or non-oxidizing atmosphere to convert the organic constituents, i.e. the organic fibers and thermoplastic binder, to carbon char. Firing the structure in the inert atmosphere eliminates gassing of the organic constituents and yields a part suitable for final binder application.
A final coating of silicon carbide is then applied to the filter structure using a conventional chemical vapor deposition process. The silicon carbide coats the haphazardly arranged ceramic fibers, as well as the junctions or intersections between the fibers. The resulting structure is a composite of ceramic fibers, inorganic binders and carbon char, coated with silicon carbide.
In a modified form of the parent invention, the green filter structure is coated with an aqueous colloidal solution of an inorganic material, such as alumina or alumina-silicate binders. The part is then air dried, heated to a temperature of about 200xc2x0 C. to 300xc2x0 C. to remove solvents and dehydrate the colloidal material and then fired at a temperature of 900xc2x0 C. to 1100xc2x0 C. in air to remove the organic components. Following this, the final coating of silicon carbide is applied using the chemical vapor deposition process.
The silicon carbide coating thickness is controlled to a level of about 0.5 to 1.5 microns, so that the porosity of the filter structure is not adversely affected and is maintained at a value of 80% void or greater. Because of the silicon carbide coating, the resulting composite filter has improved mechanical strength, 6,000 kPa or greater having been shown, which is 50-100% higher than can be achieved by bonding or sintering the fibers alone. Additionally, there is no significant degradation or loss of pores within the structure so that resistance to gas flow is minimal.
With the method of the parent invention, a green state part is produced with a production capable process and the geometry of this part is maintained throughout conversion to a high temperature ceramic composite, and this geometry will be retained at the elevated temperatures of about 650xc2x0 C. to 700xc2x0 C. needed to regenerate a contaminated filter.
The present invention arose during continuing development efforts relating to the noted parent invention. The present invention provides a green uncured pre-form for subsequent manufacture, e.g. by curing and rigidization, into an exhaust aftertreatment filter suitable preferably for use to control diesel exhaust emissions, for example diesel particulates, nitrous oxide (NOX), carbon monoxide (CO), and/or other hydrocarbon emissions. The pre-form is preferably a cylindrical, porous, ceramic structure with alternating layers of flat and pleated fibrous media. It is formed by rolling together a pleated layer of media bound to a flat layer using a suitable adhesive which facilitates fabrication and handling and ensures structural integrity of the finished filter. The channels formed by the intersection of the rolled pleated and flat layers run in an axial direction to the cylindrical structure of the pre-form along its length. Lower restriction and greater structural strength is provided, including crush strength which is desirable for packaging and sleeving the element in the finished product. Particular geometries have been found to improve performance.
The walls of the pre-form have a porosity greater than 80% and are made from fibrous filter media. In its green uncured state, the media with both pleated and flat layers contains greater than 80% by weight of fibers with suitable thermal and chemical resistance for exhaust gas temperatures and conditions, including alumina, alumina-silica, and silicon carbide. The media contains less than 20% of fugitive and other materials and fibers, as well as intermediate binders to facilitate processing by imparting wet strength, increasing tear and cut resistance, and increasing flexibility and pliability of the media during manufacturing. These fugitive and other materials may include, inter alia: synthetic fibers, microfibers and/or pulps, such as Kevlar, cellulose, acrylic, acetate, polypropylene, polyester, and nylon; organic emulsion polymer resins such as acrylic, vinyl chloride, nitrile, polyvinyl acetate, or thermosetting materials, like phenolic or epoxy; inorganic glass microfibers, that are primarily SiO2; miscellaneous chemicals, such as acid, alum, etc. to control pH and aid dispersion; colloidal cellulose, including carboxy methyl cellulose; and inorganic particles or colloidal material, such as alumina, alumina-silica, silicon carbide, rare earth and/or mixed metal oxides. Some of these materials may incinerate or change their physical form during curing to increase the porosity of the final filter product. The media thickness is less than 0.8 mm (millimeters), and, ideally should be 0.5 mm or less, in order to reduce restriction. Together, the high porosity and thin walls of the media reduce the thermal mass of the product making it easier to heat and regenerate the filter, relative to extruded monolithic structures.
When the pre-form is processed into a diesel particulate filter, it works in a wall-flow mode with alternating ends of the channels plugged in order to force the flow of exhaust gases through the porous walls of the filter. The plugging material must adhere to the media, plug the ends of the channels, and be able to survive exhaust conditions. Typically, a suspension with properties compatible with the filter media and containing metal oxides with or without silicon carbide is used. When used as a substrate for a catalyst for the removal of NOX, CO and/or hydrocarbon emissions, the ends are not plugged and the porous wall structure of the pre-form increases the surface area available for the chemical reactions.
The present invention further provides forming apparatus for the above noted pre-form and methods of configuring and shaping same.
Due to the geometry of diesel emission control filters and the properties of the media, an alternative to conventional pleating and corrugation methods is needed to optimize the noted characteristics of the diesel emission control filter. In preferred form, specifically, it is desirable that the diesel emission control filter use pleated media with either triangular or trapezoidal cross-sections, rather than corrugated media with a sinusoidal shaped cross-section. The difference is significant for greater strength, stability, and structural integrity. This is particularly desirable in the present exhaust aftertreatment filter application because force is typically applied in a radial direction to seal and hold the filter element in place. The pleat height is less than 10 mm, and preferably less than 6 mm.
Conventional methods, including score-roll pleating and corrugation, cannot produce media with the noted desired geometry and structure. Pleating is commonly done by score-rolling, wherein the media passes between two rollers with male and female spikes and slots that score the media. The media then passes through downstream gathering wheels that feed the media against an opposing force. The inherent stiffness of the media causes the media to fold or crease into pleats along the score lines. For this reason, score-roll pleating is unsuitable for pliable media with insufficient stiffness, such as the ceramic media used in the above noted diesel emission control filter. Another limitation is that the desired small pleat heights, e.g. less than 10 mm, cannot be obtained by the noted score-roll pleating method. Furthermore, creasing along the noted score lines can damage the media pleat tips for some types of filter media, including that used for the noted diesel emission control filter.
Another possibility is to use corrugation for producing a diesel emission control filter. In this method, corrugated rollers are used to imprint a shape onto the media, instead of creasing and folding the media as is done with other pleating methods. The limitation of the noted corrugation method is that the pleats have a sinusoidal cross-section, rather than triangular or trapezoidal. As above noted, triangular or trapezoidal flutes or channels are desired, with cross-sectional geometries which are more structurally stable and provide for more laminar flow.
In one aspect of the present invention, a star gear pleating method is used to produce the pleated media pre-form for a diesel engine control filter. Particularly designed interlocking gears pass the media between one or more sharp tips of a gear tooth on one gear and a particularly formed root of the opposing gear. The teeth can be modified to provide triangular or trapezoidal pleats. The gears fold and gather the media without crushing it and without adversely affecting the performance of the final filter product. As the media is released from the interlocking gears, it is directed forward and out of the gears by guide bars which prevent the media from tending to follow the gears and become damaged. The present method does not rely on media stiffness to fold and crease the media, and hence it can be used on more pliable media without damage to the pleat tips. The media is partially gathered and folded between the interlocking teeth of the gears, which partial gathering helps prevent unwanted jams or reverse pleating otherwise common with score-roll pleating. The present method and forming apparatus allows much shorter pleat heights and faster pleating without damaging the media or breaking the fibers. In contrast to corrugation, the present technique produces straight sided triangular or trapezoidal pleats. The present technique further eliminates the need to use extruded ceramic monoliths for diesel emission control filters.